1. Field of the Invention
The present invention is directed generally to the reaction that occurs between glucose and proteins, and more specifically, to the identification of a new group of compounds which form during nonenzymatic browning and whose existence became apparent by the studies of the inhibition of this reaction by aminoguanidine and its related analogs.
2. Description of the Prior Art
About eighty years ago, Louis Maillard first investigated the reaction of reducing sugars with the free amino groups of amino acids and proteins. This complicated reaction, termed the Maillard reaction, or non-enzymatic browning, is responsible for the aroma and taste in cooked or preserved foods.
This reaction is initiated by the reaction of primary amines (from amino acids, proteins and nucleic acids) with sugars to form imines (Schiff bases) which undergo a rearrangement to form Amadori products (AP) (see Scheme 1, below). Further rearrangements of the Amadori product are responsible for the browning and fluorescent products which lead to the formation of advanced glycosylation endproducts (AGEs). ##STR1##
Over the last fifteen years, it has been demonstrated that the Maillard reaction also occurs in vivo. A variety of proteins, such as hemoglobin, lens crystallins, connective tissue collagen, glomerular basement membrane and nerve tissue proteins, are modified by glucose in normal individuals. In diabetic patients the level of Amadori product is generally 2-3 fold higher than non-diabetics. Thus, the advanced glycosylation endproducts from Amadori product play an important role in the pathogenesis of the complications in diabetes and aging.
The chemistry of AGE formation is not yet well understood because of its extreme complexity. A variety of Maillard compounds have been identified from incubations prepared at high temperature or under acidic conditions. There is little structural information about Maillard compounds which form under physiological conditions. In 1986, Brownlee et al. found aminoguanidine to be an effective inhibitor of advanced glycosylation. Later it was shown that aminoguanidine-treated diabetic animals had significantly less collagen-linked fluorescence and crosslinking in the vascular wall than the untreated ones. Presently, aminoguanidine has displayed excellent efficacy in treating a variety of diabetic complications and is undergoing clinical efficacy trials.
The precise mechanism by which aminoguanidine and its analogs prevent advanced glycosylation is unknown. Several deoxydiketoses are detected as the degradation products of the Amadori products at various pHs and at elevated temperature, for example, 1-deoxyglucosone, 3-deoxyglucosone and 4-deoxyglucosone (see Scheme 2 below). In most studies to date, these compounds have been obtained under non-physiological conditions (high concentrations, low pH and high temperature). For example, 3-deoxyglucosone was formed preferably under acidic conditions. Furthermore, these compounds are very unstable and proof of their existence is often via chemical derivatization. ##STR2##